"I can't believe it's been a year."

One year later, Carlos Morales is still hanging in there. The father to quadruplets tragically lost his significant other during childbirth one year ago, but has managed to keep a positive outlook on an ordeal he admits is “bittersweet.” January 15 marks his four kids’ first birthday, but also the death of their mom, Erica, who died from hypovolemic shock.

The day was filled with laughter, friends and family, but it was also a reminder of what has been painfully missing since his four kids were born.

Carlos Jr., identical twins Erica and Paisley and Tracy – were born at 5:51 p.m. on Jan. 15, 2015. Just eight hours after they arrived, Morales’s wife Erica died from hypovolemic shock, an emergency condition involving massive blood loss.

“It’s all very bittersweet,” Morales of Phoenix, Arizona, tells PEOPLE with tears in his eyes. “I’m celebrating their birthday, but I also lost my wife and they lost their mom.”

Celebrating their big day with a lot of balloons, birthday cake, friends, and family, Carlos said, “Erica would have loved this. It would have been an even bigger party. She really liked this kind of stuff. She was looking forward to it.”

He also talks about not having had much time to grieve:

“I’m rarely alone,” he says. “I can’t believe it’s been a year. I just wish Erica was here to see them crawl and see them start to walk. There are going to be so many more firsts she won’t be around for.”